As he selected a recent map from one of the pigeonholes, Murena reflected on the confidence of Severus. When, not if he had rescued the prince. Murena doubted that the expedition would get anywhere near the Castle of Silence.

With the map spread on the desk, they stood looking down at the little depictions of towns, the lines representing roads, the imprecise drawings of mountains, rivers and seas.

Volusianus pointed to the great emptiness of the Steppes. His finger moved from north-east of the Black Sea to east of the Caspian. ‘All this territory is ruled by Naulobates, King of the Heruli. Since the mission of Ballista two years ago, the King has been recognised as a friend and ally of the Roman people. Naulobates will be informed of your coming.’

‘Sir,’ Severus cleared his throat, ‘might it not be better to send a man known to this Naulobates?’

‘I have other plans for Ballista.’ Volusianus dismissed the idea. ‘When you have the prince, you will ride due north, the Caspian on your left, until you reach the Heruli.’

His fate sealed, Severus nodded and turned to practicalities. ‘Where will I rendezvous with the frumentarii?’

Volusianus jabbed at the map. ‘At Zeugma on the Euphrates. There you will cross from Syria into Mesopotamia.’

Murena considered the small town on the wide, brown river. He had been born there, the son of a soldier with the Fourth Scythian Legion. As a boy he had seen the army of the Emperor Alexander Severus straggle back across the bridge from defeat at the hands of the Persians. As a man enrolled in the same legion, he had fought in the vast wastes of Mesopotamia. Although the towns there were now held by the Romans, most had been sacked at one time or another. When Zeugma itself fell, he had barely escaped with his life. Only the gods had preserved him when the Emperor Valerian had been captured near Carrhae. The bare countryside of Mesopotamia was a no man’s land, subjected to endless raids by Persians and nomadic Arabs. Severus and his rescue mission would be lucky to get that far, let alone reach the distant Castle of Silence.

The Praetorian Prefect and Severus had turned to a discussion of the merchandise the caravan should take. As they talked, Murena mulled over the vital question that had not been asked. Why should Volusianus want to rescue Prince Sasan?

It was well known that the Praetorian Prefect had long urged the Emperor Gallienus to lead an expedition to free his father Valerian from Persian captivity. Gallienus had proved reluctant. If Prince Sasan fell into Roman hands, war would be inevitable. Even if he wished, Shapur would not be able to ignore the blow to his prestige as King of Kings. Gallienus would have to respond. Volusianus would get his desire. And his influence over the Emperor would be demonstrated.